[fuel-core] Update Content Type regex to better match spec

[ProjectZed]

Description

Replaced the required space regex with an optional space regex.
Fixes #671

Type of change

Check all that apply

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactoring (a change which changes the current internal or external interface)
• This change requires a documentation update

How Has This Been Tested?

Matching "application/json; charset=utf-8" and "application/json;charset=utf-8" with a regex tester
https://regex101.com/r/tpBlL3/1

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation, if necessary
• My changes generate no new compiler warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Inspect the bytecode viewer, including reasoning why

[codecov]

Codecov Report

Merging #673 into master will not change coverage.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #673   +/-   ##
=========================================
  Coverage     71.32%   71.32%           
  Complexity      296      296           
=========================================
  Files            58       58           
  Lines          1496     1496           
  Branches        194      194           
=========================================
  Hits           1067     1067           
  Misses          334      334           
  Partials         95       95

Impacted Files	Coverage Δ	Complexity Δ
...m/github/kittinunf/fuel/core/BodyRepresentation.kt	75% <100%> (ø)	0 <0> (ø)	⬇️

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[SleeplessByte]

You're the best! Thank you ❤️

[kittinunf]

@ProjectZed 🍻