Change DIDUrl::join to borrow self

[cycraig]

Description of change

Changes DIDUrl::join to borrow self instead of consuming it, as discussed in #805.

Technically a breaking change since explicit functions calls with e.g. DIDUrl::join(url, "blah") would need to change to e.g. DIDUrl::join(&url, "blah").

Links to any relevant issues

See discussion #805.

Type of change

• Bug fix (a non-breaking change which fixes an issue)
• Enhancement (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Fix

How the change has been tested

Minor change, existing unit tests cover it.

Change checklist

• I have followed the contribution guidelines for this project
• 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
• 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

[olivereanderson]

LGTM