Fix url encoding of list parameters

[isra17]

Issue can easily be reproduce with API using list parameters such as testIamPermissions in Cloud Storage.
Before the fix, using a list end up with this error:

bucket.test_iam_permissions(['storage.buckets.delete','storage.buckets.get'])
# Raises
# google.cloud.exceptions.BadRequest: 400 null is not a valid Google Cloud Storage permission. (GET https://www.googleapis.com/storage/v1/b/some-buckets/iam/testPermissions?permissions=%5B%27storage.buckets.delete%27%2C+%27storage.buckets.get%27%5D)

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[isra17]

I signed it!

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CLAs look good, thanks!

[tseaver]

@isra17 Thanks for the patch! Can you please add a unit test (like the one in your example above) which fails when 'query_params' contains a list value?

[isra17]

Couldn't really add a test from my initial example since API tests rely on mocking, but I modified the _http test.

[tseaver]

@isra17 Thank you very much!