Add basic HttpRequestEntity implementations. Implements #106

[dmfs]

This adds

• JsonRequestEntity to send JSONObjects or JSONArrays
• TextRequestEntity to send plain text
• XWwwFormUrlEncodedEntity to send x-www-form-urlencoded key-value pairs

[codecov-io]

Codecov Report

Merging #107 into master will increase coverage by 0.45%. The diff coverage is 93.1%.

@@             Coverage Diff             @@
##             master    #107      +/-   ##
===========================================
+ Coverage     66.64%   67.1%   +0.45%     
- Complexity      477     483       +6     
===========================================
  Files           144     147       +3     
  Lines          1649    1678      +29     
  Branches        112     112              
===========================================
+ Hits           1099    1126      +27     
- Misses          497     499       +2     
  Partials         53      53

Impacted Files	Coverage Δ	Complexity Δ
...pessentials/entities/XWwwFormUrlEncodedEntity.java	100% <100%> (ø)	2 <2> (?)
...mfs/httpessentials/entities/JsonRequestEntity.java	100% <100%> (ø)	3 <3> (?)
...mfs/httpessentials/entities/TextRequestEntity.java	66.66% <66.66%> (ø)	1 <1> (?)

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