Previously, HttpClient used the overly clever test "body || null"
to determine when a body parameter was provided. This breaks when
the valid bodies '0' or 'false' are provided.
This change tests directly against 'undefined' to detect the presence
of the body parameter, and thus correctly allows falsy values through.
Fixes#19825.
Fixes#19195.
PR Close#19958
HttpClient is an evolution of the existing Angular HTTP API, which exists
alongside of it in a separate package, @angular/common/http. This structure
ensures that existing codebases can slowly migrate to the new API.
The new API improves significantly on the ergonomics and features of the legacy
API. A partial list of new features includes:
* Typed, synchronous response body access, including support for JSON body types
* JSON is an assumed default and no longer needs to be explicitly parsed
* Interceptors allow middleware logic to be inserted into the pipeline
* Immutable request/response objects
* Progress events for both request upload and response download
* Post-request verification & flush based testing framework
Alex Rickabaugh