Add Request interface and rename Response::getBody()

[Art4]

This PR modifies the HttpClient::request() method to allow handling implementation of the new Request interface, described in #341. It also renames Response::getBody() to Response::getContent(), see #363.

In the tests I introduced an AssertingHttpClient that allows us easily define a HttpClient with asserted Requests and expected Responses. This will become very handy while starting deprecating the Redmine\Client\Client interface and replacing it in tests.

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (c06b115) 96.84% compared to head (a9fb021) 96.80%.

Files	Patch %	Lines
src/Redmine/Api/AbstractApi.php	96.92%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               v2.x     #366      +/-   ##
============================================
- Coverage     96.84%   96.80%   -0.04%     
- Complexity      550      556       +6     
============================================
  Files            27       27              
  Lines          1523     1567      +44     
============================================
+ Hits           1475     1517      +42     
- Misses           48       50       +2     

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