Allow CURL constant names (php-http#82)

Allow CURL constant names and validate we found them

ClientFactory/CurlFactory.php

@@ -40,6 +40,19 @@ public function createClient(array $config = [])
             throw new \LogicException('To use the Curl client you need to install the "php-http/curl-client" package.');
         }
 
+        // Try to resolve curl constant names
+        foreach ($config as $key => $value) {
+            // If the $key is a string we assume it is a constant
+            if (is_string($key)) {
+                if (null === ($constantValue = constant($key))) {
+                    throw new \LogicException(sprintf('Key %s is not an int nor a CURL constant', $key));
+                }
+
+                unset($config[$key]);
+                $config[$constantValue] = $value;
+            }
+        }
+
         return new Client($this->messageFactory, $this->streamFactory, $config);
     }
 }