Fix deprecated Guzzle functions.

* Replace query builder function with method on Query object.
* Added explicit leading '/' on every path due to guzzle deprecatino
  warning.

src/Request/Models/Endpoint.php

@@ -9,9 +9,7 @@
 use Consilience\Starling\Payments\HydratableTrait;
 use Consilience\Starling\Payments\ValidationTrait;
 use Consilience\Starling\Payments\ModelInterface;
-use UnexpectedValueException;
-
-use function GuzzleHttp\Psr7\build_query;
+use GuzzleHttp\Psr7\Query;
 use GuzzleHttp\Psr7\Uri;
 
 class Endpoint implements ModelInterface
@@ -98,11 +96,15 @@ public function getUrl($servicePath = '', $query = [])
             $replacements['{' . $fieldName . '}'] = $this->getProperty($fieldName);
         }
 
-        $path = str_replace(
-            array_keys($replacements),
-            array_values($replacements),
-            $template
-        );
+        // A URL with an authority must start with a slash.
+
+        $path = '/' . ltrim(
+            str_replace(
+                array_keys($replacements),
+                array_values($replacements),
+                $template
+            ),
+        '/');
 
         $uri = new Uri();
 
@@ -112,7 +114,7 @@ public function getUrl($servicePath = '', $query = [])
             ->withHost($this->host);
 
         if (! empty($query)) {
-            $uri = $uri->withQuery(build_query($query));
+            $uri = $uri->withQuery(Query::build($query));
         }
 
         return $uri;

tests/Response/BusinessInformationTest.php

@@ -61,6 +61,7 @@ public function testFromPSR7($contentType)
                 "minorUnits": 12345
             }
         }';
+
         $businessInformationResponse = new Psr7Response(
             200,
             [