Fixed the wrong trimming of line break at at the start of code blocks.

Author: fredck

src/commonmark-rules.js

@@ -128,7 +128,7 @@ rules.fencedCodeBlock = {
 
     return (
       '\n\n' + fence + language + '\n' +
-      code.replace(/^\n|\n$/g, '') +
+      code.replace(/\n$/, '') +
       '\n' + fence + '\n\n'
     )
   }

test/index.html

@@ -904,8 +904,7 @@ <h2>This is a header.</h2>
 
 <div class="case" data-name="triple tildes inside code" data-options='{"codeBlockStyle": "fenced", "fence": "~~~"}'>
   <div class="input">
-<pre><code>
-~~~
+<pre><code>~~~
 Code
 ~~~
 </code></pre>
@@ -919,8 +918,7 @@ <h2>This is a header.</h2>
 
 <div class="case" data-name="triple ticks inside code" data-options='{"codeBlockStyle": "fenced", "fence": "```"}'>
   <div class="input">
-<pre><code>
-```
+<pre><code>```
 Code
 ```
 </code></pre>
@@ -934,8 +932,7 @@ <h2>This is a header.</h2>
 
 <div class="case" data-name="four ticks inside code" data-options='{"codeBlockStyle": "fenced", "fence": "```"}'>
   <div class="input">
-<pre><code>
-````
+<pre><code>````
 Code
 ````
 </code></pre>
@@ -947,6 +944,20 @@ <h2>This is a header.</h2>
 `````</pre>
 </div>
 
+<div class="case" data-name="empty line in start/end of code block" data-options='{"codeBlockStyle": "fenced", "fence": "```"}'>
+  <div class="input">
+<pre><code>
+Code
+
+</code></pre>
+  </div>
+  <pre class="expected">```
+
+Code
+
+```</pre>
+</div>
+
 <!-- /TEST CASES -->
 
 <script src="turndown-test.browser.js"></script>