remove undesired character "=" appearing in signed_gid to_param result

[formigarafa]

SignedGlobalID#to_param currently may produce strings containing the character "=".
In the unsigned version to_param and parse removes and deals with their absence respectively making their us in url_params a straightforward experience.
The same does not happen with their signed counterpart which its use is encouraged for such cases.

Todo:

• fix ;)

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[formigarafa]

@jeremy ,
this comment:

globalid/lib/global_id/global_id.rb

Line 68 in c41b321

# remove the = padding character for a prettier param -- it'll be added back in parse_encoded_gid

led me to do this pull request. I overlook one detail in my use case and what seemed to be caused by url encoding issued was only a mistake on a test setup.
This pull request is not necessary to make it work, only make the urls "prettier" as the comment states by removing the %3D occurrences in the middle of the ActiveSupport::MessageVerifier generated string.