Add missing User-Agent header.

[aontas]

When moving to OKHTTP3, the User-Agent header was not added to the set of headers sent to the interceptor. It previously was there.

This simply re-adds it so that it is the same as it was before.

[codecov]

Codecov Report

Merging #651 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@             Coverage Diff              @@
##             master     #651      +/-   ##
============================================
- Coverage     52.86%   52.86%   -0.01%     
  Complexity     1676     1676              
============================================
  Files           131      131              
  Lines         10146    10147       +1     
  Branches       1408     1408              
============================================
  Hits           5364     5364              
- Misses         4339     4340       +1     
  Partials        443      443

Impacted Files	Coverage Δ	Complexity Δ
Parse/src/main/java/com/parse/ParsePlugins.java	43.29% <0%> (-0.46%)	12 <0> (ø)

---

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 b80f25f...7380a85. Read the comment docs.

[rogerhu]

Thanks somehow it got deleted in 6cfc4a0

[rogerhu]

Are there any other headers too?

[aontas]

The HEADER_OS_VERSION also seems to be missing.