fix: Align metadata extraction with protocol

[ankitpokhrel]

This PR make some changes on metadata extraction to align with the protocol.

• The value MAY be empty. In these cases, the space, which would normally separate the key and the value, MAY be left out.
• Even though the protocol doesn't say anything about empty space after comma and key, the official implementation seems to trim extra spaces.

Ref: https://tus.io/protocols/resumable-upload.html#upload-metadata

Fixes #378

[codecov]

Codecov Report

Merging #379 (f9fd90c) into main (c6d98d6) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               main     #379   +/-   ##
=========================================
  Coverage     99.58%   99.58%           
  Complexity      360      360           
=========================================
  Files            20       20           
  Lines           964      968    +4     
=========================================
+ Hits            960      964    +4     
  Misses            4        4           

Impacted Files	Coverage Δ
src/Request.php	100.00% <100.00%> (ø)

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