Set base_url default value inside _request() instead of in signature

[kerrick-lyft]

If an end-user of the library wants to override _DEFAULT_BASE_URL (e.g. in order to use a proxy), this makes it easier. The user can simply run

googlemaps.client._DEFAULT_BASE_URL = 'http://my_proxy_dns'

in their server/script's startup code.

Without this change, the default value for the parameter gets "baked" into the function definition and is harder to change.

(Obviously a module's internal implementation details are subject to change, and this PR doesn't construe a guarantee that overriding _DEFAULT_BASE_URL will continue to work in the future).

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[kerrick-lyft]

I signed it!

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CLAs look good, thanks!

[kerrick-lyft]

Hey @stephenmcd could I get a review on this?

[kerrick-lyft]

It's better to properly expose this as an argument to the Client constructor. Will open a PR for that.

[thatguysimon]

Hey @kerrick-lyft, I see you didn't create another PR. Did you find another workaround for this? I'm having the same issue.