Pass a `GP_Project` to `gp_url_project()`

[swissspidy]

Fixes #102.

Description Makes sure the correct type is passed to the function.

Types of changes Bug fix.

Checklist:

• [x] My code is tested.
• [x] My code follows the WordPress code style.
• [x] My code has proper inline documentation.

[codecov-io]

Codecov Report

Merging #106 into master will not change coverage. The diff coverage is 0%.

@@            Coverage Diff            @@
##             master     #106   +/-   ##
=========================================
  Coverage     74.81%   74.81%           
  Complexity      331      331           
=========================================
  Files            23       23           
  Lines           798      798           
=========================================
  Hits            597      597           
  Misses          201      201

Impacted Files	Coverage Δ	Complexity Δ
inc/Plugin.php	24.8% <0%> (ø)	39 <0> (ø)	:arrow_down:

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