fixing downgrade of MAX_CONCURRENT_STREAMS

in a situation where this was set by default to 200, and when receiving
settings with a lower value (let's say, 100), the connection didn't know
what to do with the remaining 100, and this was left hanging; now, it
signalizes the session that we need an extra connection, and this will
pick up the remaining and establish a new connection.

lib/httpx/connection/http2.rb

@@ -211,6 +211,7 @@ module HTTPX
 
       close
       emit(:close)
+      emit(:exhausted) unless @pending.empty?
     end
 
     def on_frame(bytes)

lib/httpx/session.rb

@@ -78,7 +78,10 @@ module HTTPX
       connection.on(:exhausted) do
         other_connection = connection.create_idle
         other_connection.merge(connection)
-        pool.init_connection(other_connection, options)
+        catch(:coalesced) do
+          pool.init_connection(other_connection, options)
+        end
+        set_connection_callbacks(other_connection, connections, options)
         connections << other_connection
       end
     end