[perf] Reduce buffer allocations (#2000)

Do not convert strings to `Buffer`s if data does not need to be masked.

Refs: #1998

Author: lpinca

lib/permessage-deflate.js

@@ -313,7 +313,7 @@ class PerMessageDeflate {
   /**
    * Compress data. Concurrency limited.
    *
-   * @param {Buffer} data Data to compress
+   * @param {(Buffer|String)} data Data to compress
    * @param {Boolean} fin Specifies whether or not this is the last fragment
    * @param {Function} callback Callback
    * @public
@@ -395,7 +395,7 @@ class PerMessageDeflate {
   /**
    * Compress data.
    *
-   * @param {Buffer} data Data to compress
+   * @param {(Buffer|String)} data Data to compress
    * @param {Boolean} fin Specifies whether or not this is the last fragment
    * @param {Function} callback Callback
    * @private