fix(common): prevent repeated application of HttpParams mutations (#29045)

Previously, an instance of HttpParams would retain its list of mutations
after they have been materialized as a result of a read operation. Not
only does this unnecessarily hold onto memory, more importantly does it
introduce a bug where branching of off a materialized instance would
reconsider the set of mutations that had already been applied, resulting
in repeated application of mutations.

This commit fixes the bug by clearing the list of pending mutations
after they have been materialized, such that they will not be considered
once again for branched off instances.

Fixes #20430

PR Close #29045

packages/common/http/src/params.ts

@@ -227,7 +227,7 @@ export class HttpParams {
             }
         }
       });
-      this.cloneFrom = null;
+      this.cloneFrom = this.updates = null;
     }
   }
 }

packages/common/http/test/params_spec.ts

@@ -53,6 +53,15 @@ import {HttpParams} from '@angular/common/http/src/params';
         const mutated = body.delete('a', '2').delete('a', '4');
         expect(mutated.getAll('a')).toEqual(['1', '3', '5']);
       });
+
+      it('should not repeat mutations that have already been materialized', () => {
+        const body = new HttpParams({fromString: 'a=b'});
+        const mutated = body.append('a', 'c');
+        expect(mutated.toString()).toEqual('a=b&a=c');
+        const mutated2 = mutated.append('c', 'd');
+        expect(mutated.toString()).toEqual('a=b&a=c');
+        expect(mutated2.toString()).toEqual('a=b&a=c&c=d');
+      });
     });
 
     describe('read operations', () => {