backendSrv: Only stringifies request body if payload isn't already a string (#21639)

* backendSrv: Only stringifies request body if payload isn't already a string

* Add test cases to verify behaviour

public/app/core/services/backend_srv.ts

@@ -474,7 +474,7 @@ export class BackendSrv implements BackendService {
       Accept: 'application/json, text/plain, */*',
       Accept: 'application/json, text/plain, */*',
       ...options.headers,
       ...options.headers,
     },
     },
-    body: JSON.stringify(options.data),
+    body: typeof options.data === 'string' ? options.data : JSON.stringify(options.data),
   });
   });
 
 
   private getFromFetchStream = (options: BackendSrvRequest) => {
   private getFromFetchStream = (options: BackendSrvRequest) => {

public/app/core/specs/backend_srv.test.ts

@@ -160,7 +160,8 @@ describe('backendSrv', () => {
       ${'GET'}     | ${undefined}             | ${undefined}                       | ${{ method: 'GET', headers: { 'Content-Type': 'application/json', Accept: 'application/json, text/plain, */*' }, body: undefined }}
       ${'GET'}     | ${undefined}             | ${undefined}                       | ${{ method: 'GET', headers: { 'Content-Type': 'application/json', Accept: 'application/json, text/plain, */*' }, body: undefined }}
       ${'GET'}     | ${{ Auth: 'Some Auth' }} | ${undefined}                       | ${{ method: 'GET', headers: { 'Content-Type': 'application/json', Accept: 'application/json, text/plain, */*', Auth: 'Some Auth' }, body: undefined }}
       ${'GET'}     | ${{ Auth: 'Some Auth' }} | ${undefined}                       | ${{ method: 'GET', headers: { 'Content-Type': 'application/json', Accept: 'application/json, text/plain, */*', Auth: 'Some Auth' }, body: undefined }}
       ${'GET'}     | ${{ Auth: 'Some Auth' }} | ${{ data: { test: 'Some data' } }} | ${{ method: 'GET', headers: { 'Content-Type': 'application/json', Accept: 'application/json, text/plain, */*', Auth: 'Some Auth' }, body: '{"data":{"test":"Some data"}}' }}
       ${'GET'}     | ${{ Auth: 'Some Auth' }} | ${{ data: { test: 'Some data' } }} | ${{ method: 'GET', headers: { 'Content-Type': 'application/json', Accept: 'application/json, text/plain, */*', Auth: 'Some Auth' }, body: '{"data":{"test":"Some data"}}' }}
-      ${'GET'}     | ${{ Auth: 'Some Auth' }} | ${'some data'}                     | ${{ method: 'GET', headers: { 'Content-Type': 'application/json', Accept: 'application/json, text/plain, */*', Auth: 'Some Auth' }, body: '"some data"' }}
+      ${'GET'}     | ${{ Auth: 'Some Auth' }} | ${'some data'}                     | ${{ method: 'GET', headers: { 'Content-Type': 'application/json', Accept: 'application/json, text/plain, */*', Auth: 'Some Auth' }, body: 'some data' }}
+      ${'GET'}     | ${{ Auth: 'Some Auth' }} | ${'{"data":{"test":"Some data"}}'} | ${{ method: 'GET', headers: { 'Content-Type': 'application/json', Accept: 'application/json, text/plain, */*', Auth: 'Some Auth' }, body: '{"data":{"test":"Some data"}}' }}
     `(
     `(
       "when called with method: '$method', headers: '$headers' and data: '$data' then result should be '$expected'",
       "when called with method: '$method', headers: '$headers' and data: '$data' then result should be '$expected'",
       ({ method, headers, data, expected }) => {
       ({ method, headers, data, expected }) => {