Single mapImage shouldn’t be optional

[bjarkehs]

Unless I misunderstand how RxSwift works then the signature for Single's mapImage is incorrect. The comment also states that if conversion fails, then the signal errors. So the value will always be non-optional.

Response's mapImage also returns non-optional UIImage.

[MoyaBot]

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[codecov-io]

Codecov Report

Merging #1789 into development will not change coverage.
The diff coverage is 100%.

@@             Coverage Diff              @@
##           development    #1789   +/-   ##
============================================
  Coverage        90.83%   90.83%           
============================================
  Files                5        5           
  Lines              131      131           
============================================
  Hits               119      119           
  Misses              12       12

Impacted Files	Coverage Δ
Sources/RxMoya/Single+Response.swift	100% <100%> (ø)	⬆️

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[codecov-io]

Codecov Report

Merging #1789 into development will not change coverage.
The diff coverage is 100%.

@@             Coverage Diff              @@
##           development    #1789   +/-   ##
============================================
  Coverage        90.83%   90.83%           
============================================
  Files                5        5           
  Lines              131      131           
============================================
  Hits               119      119           
  Misses              12       12

Impacted Files	Coverage Δ
Sources/RxMoya/Single+Response.swift	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[sunshinejr]

Sorry for late merge & thanks for the work @bjarkehs!👍