Add HttpUtil test

[chanseokoh]

Part of #1437.

[elharo]

HashMap<>()

[chanseokoh]

I realized I can't use the diamond syntax when I'm using the double-brace initialization. I could have used Guava, but this bundle is free of Guava, so I decided not to do so.

Now I think I'll change this to plain static initialization, because the double-brace initialization gives me the serial ID warning.

[akerekes]

LGTM once the builds are successful

[codecov-io]

Codecov Report

Merging #1535 into master will increase coverage by 0.15%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1535      +/-   ##
============================================
+ Coverage     70.31%   70.47%   +0.15%     
+ Complexity     1302     1301       -1     
============================================
  Files           232      232              
  Lines          8942     8942              
  Branches        761      761              
============================================
+ Hits           6288     6302      +14     
+ Misses         2338     2321      -17     
- Partials        316      319       +3

Impacted Files	Coverage Δ	Complexity Δ
...clipse/appengine/facets/NonSystemJobSuspender.java	83.78% <0%> (-5.41%)	9% <0%> (-1%)
.../tools/eclipse/test/util/project/ProjectUtils.java	79.59% <0%> (-1.03%)	18% <0%> (-1%)
...d/tools/eclipse/test/util/http/TestHttpServer.java	93.87% <0%> (+2.04%)	12% <0%> (ø)	❌
...gle/cloud/tools/eclipse/usagetracker/HttpUtil.java	89.79% <0%> (+32.65%)	7% <0%> (+1%)	✅

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 78772ab...7e57beb. Read the comment docs.