bug: number of channels was inadvertently limited to 1 (googleapis/google-cloud-cpp-spanner#1169)
bug: number of channels was inadvertently limited to 1 (googleapis/google-cloud-cpp-spanner#1169)

* bug: number of channels was inadvertently limited to 1

The intent was to ensure the number of channels was "at least one",
but instead actually made it "at most one".

There is no test for this because client_test uses the Client
constructor directly. We should try to add one, but let's fix
this obvious bug first.

googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp