Ensure we find the filename in Upload-Metadata when clients send extra

[drastik]

I wrote a Drupal integration (https://www.drupal.org/project/tus), and we need a few extra values passed from Uppy in the meta{} option.
entityType, entityBundle, fieldName (for determining folder storage based on user's settings within the CMS).

tus-php assumes a single value, or that the first value in the Upload-Metadata header is filename.
When I plug in the extra values in Uppy, filename occurs last, and tus-php fails to find filename properly.

This will fix that.

[drastik]

@ankitpokhrel Is this good to go?

Let me know if you want any code style changes, this is a very important patch though.

[ankitpokhrel]

@drastik tests are failing and also I am not ok with the changes.

For me there should be a separate method say extractMeta or similar that will take the key and return value for that key. It should follow rules for upload meta mentioned in tus protocol. extractFilename can then use extractMeta method to return file name.

The Upload-Metadata request and response header MUST consist of one or more comma-separated 
key-value pairs. The key and value MUST be separated by a space. The key MUST NOT contain 
spaces and commas and MUST NOT be empty. The key SHOULD be ASCII encoded and the value 
MUST be Base64 encoded. All keys MUST be unique.

[ankitpokhrel]

uppy uses name as key instead of filename so we can't just use filename directly

[samundra]

I will address the changes that @ankitpokhrel has asked for. I have a hunch what needs to be done and will fix the breaking tests as well. Besides, I will try to have a look into drupal module as well and provide feedback by tonight. Happy Hacktoberfest guys.

[ankitpokhrel]

Fixed in #84