Allow proxy url with parameters

[mzand33]

Summary
Currently, proxy url's cannot have parameters.

This PR fixes/implements the following bugs/features
2098

Explain the motivation for making this change. What existing problem does the pull request solve?

If a proxy url contains a parameter such as:
http://abc.com/myproxy.jsp?myparam=foo
html2canvas will call the proxy using the following url:
http://abc.com/myproxy.jsp?myparam=foo?url=abc.com....
Notice there are two question marks in the url which is incorrect. The second parameter should begin with an ampersand.
The code had automatically put a question mark (?) after the proxy url. I fixed the code to check whether there is already a parameter at the end of the url. This was properly done in the the 0.5 version:

Test plan (required)
Pass a proxy url which ends with a parameter(s) as an option.

Code formatting

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Closing issues

Fixes #2098
Closes #2098

[Akshay-Bindal]

What is the status of this PL? Can we please expedite this as it seems like a common use case where proxy url can have existing query params.