Fixes memory exhaustion when downloading big files

Makes parsing the body of a response (as JSON) a lazy operation, rather
than eager. Parsing the body causes the entire response to be loaded
into memory at once, which can lead to memory exhaustion when attempted
on very large responses (e.g., the binary contents of a very large
file).

Fixes #18

lib/ApiResponse.php

@@ -14,22 +14,36 @@ class ApiResponse
     private $body;
     private $code;
     private $headers;
+    private $parsedBody;
 
     public function __construct($body, $code, $headers)
     {
-        $this->body = json_decode($body, true);
+        $this->body = $body;
         $this->code = $code;
         $this->headers = $headers;
     }
 
+    /**
+     * Get the body of the response. Returns an array if the body is valid JSON.
+     *
+     * Note that this method will attempt to convert the entire request body to a string in memory. Callers must
+     * ensure that the content length of the response is not so large that it would cause memory exhaustion. Use
+     * getBodyRaw() to get the raw body without loading its entire contents into memory.
+     *
+     * @return array|null
+     */
     public function getBody()
     {
-        return $this->body;
+        if (is_null($this->parsedBody)) {
+            $this->parsedBody = json_decode($this->body, true);
+        }
+
+        return $this->parsedBody;
     }
 
     public function getBodyRaw()
     {
-        return json_encode($this->body);
+        return $this->body;
     }
 
     public function getCode()