fix: contentType in StringEncoder should specify charset=utf-8

[lxohi]

in order to avoid encoding detection issues in web browsers

[codecov-io]

Codecov Report

Merging #672 into develop will decrease coverage by 0.18%. The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop     #672      +/-   ##
===========================================
- Coverage    85.85%   85.67%   -0.19%     
===========================================
  Files          105      105              
  Lines         4497     4524      +27     
  Branches       426      424       -2     
===========================================
+ Hits          3861     3876      +15     
- Misses         636      648      +12

Impacted Files	Coverage Δ
...rc/main/scala/colossus/protocols/http/Header.scala	86.85% <100%> (+0.07%)	:arrow_up:
.../main/scala/colossus/protocols/http/HttpBody.scala	90.47% <100%> (ø)	:arrow_up:
...cala/colossus/protocols/memcache/MemcacheOps.scala	66.66% <0%> (-12.29%)	:arrow_down:
...lossus/src/main/scala/colossus/core/Pipeline.scala	95.91% <0%> (-2.05%)	:arrow_down:
...main/scala/colossus/streaming/CircuitBreaker.scala	86.53% <0%> (-1.93%)	:arrow_down:
colossus/src/main/scala/colossus/core/Worker.scala	77.33% <0%> (-1.78%)	:arrow_down:
.../scala/colossus/metrics/collectors/Histogram.scala	91.3% <0%> (+0.72%)	:arrow_up:
...n/scala/colossus/protocols/memcache/Memcache.scala	93.65% <0%> (+1.12%)	:arrow_up:

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[yahoocla]

Thank you for submitting this pull request, however I do not see a valid CLA on file for you. Before we can merge this request please visit https://yahoocla.herokuapp.com/ and agree to the terms. Thanks! :smile:

[lxohi]

close for trigger the re-check of yahoocla

[dkarivalis]

possibly try to re-sign the CLA? CLA web hook is reporting No CLA found for this user

[lxohi]

Yes, I've changed charset to uppercase & re-signed the CLA.