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.

Author: mr-salty

google/cloud/spanner/client.cc

@@ -246,7 +246,7 @@ std::shared_ptr<Connection> MakeConnection(
     std::unique_ptr<RetryPolicy> retry_policy,
     std::unique_ptr<BackoffPolicy> backoff_policy) {
   std::vector<std::shared_ptr<internal::SpannerStub>> stubs;
-  int num_channels = std::min(connection_options.num_channels(), 1);
+  int num_channels = std::max(connection_options.num_channels(), 1);
   stubs.reserve(num_channels);
   for (int channel_id = 0; channel_id < num_channels; ++channel_id) {
     stubs.push_back(