Fix compiler warnings - remove usage of String.characters

[codecov-io]

Codecov Report

Merging #111 into master will decrease coverage by 0.52%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #111      +/-   ##
==========================================
- Coverage   49.49%   48.97%   -0.53%     
==========================================
  Files          23       23              
  Lines        1388     1707     +319     
  Branches        0       53      +53     
==========================================
+ Hits          687      836     +149     
- Misses        701      846     +145     
- Partials        0       25      +25

Impacted Files	Coverage Δ
Source/StringExtension.swift	85.71% <100%> (ø)	:arrow_up:
Source/RSCode39Generator.swift	87.09% <100%> (+0.73%)	:arrow_up:
Source/RSExtendedCode39Generator.swift	50% <100%> (+2.34%)	:arrow_up:
Source/UIColorExtension.swift	32.89% <100%> (+5.91%)	:arrow_up:
Source/RSCode39Mod43Generator.swift	100% <100%> (ø)	:arrow_up:
Source/RSCode93Generator.swift	90.32% <100%> (+1.19%)	:arrow_up:
.../RSBarcodesSampleTests/RSBarcodesSampleTests.swift	76.54% <0%> (-23.46%)	:arrow_down:
...RSBarcodesSample/BarcodeReaderViewController.swift	40.54% <0%> (-4.46%)	:arrow_down:
Source/RSUnifiedCodeGenerator.swift	76.56% <0%> (-3.44%)	:arrow_down:
...SBarcodesSample/RSBarcodesSample/AppDelegate.swift	36.36% <0%> (-1.14%)	:arrow_down:
... and 18 more

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[neoneye]

I just tried downloading this PR and compiling using Xcode9.2, but I'm seeing these warnings

[tomaskraina]

1. Yes, that's expected. I didn't migrate it to Swift 4, this is still Swift 3.2 code.
2. I can fix the "Update to recommended settings" warning but it will have no effect on anything as long as you're using cocoapods to integrate of this framework.