fix: strip the newline right after the <pre> start tag

Fixes #244

src/Converter/PreformattedConverter.php

@@ -15,6 +15,10 @@ class PreformattedConverter implements ConverterInterface
         \assert($preContent !== null);
         $preContent = \str_replace('</pre>', '', $preContent);
 
+        // A newline right after the <pre> start tag is not part of the content (HTML spec), but the parser keeps it
+        $preContent = \preg_replace('/^(?:\r\n|\r|\n)/', '', $preContent);
+        \assert($preContent !== null);
+
         /*
          * Checking for the code tag.
          * Usually pre tags are used along with code tags. This conditional will check for already converted code tags,

tests/HtmlConverterTest.php

@@ -293,13 +293,15 @@ EOT;
         $this->assertHtmlGivesMarkdown('<pre>  one line with spaces  </pre>', "```\n  one line with spaces  \n```");
         $this->assertHtmlGivesMarkdown('<pre></pre>', "```\n```");
         $this->assertHtmlGivesMarkdown('<pre></pre><pre></pre>', "```\n```\n\n```\n```");
-        $this->assertHtmlGivesMarkdown("<pre>\n</pre>", "```\n\n```");
+        $this->assertHtmlGivesMarkdown("<pre>\n</pre>", "```\n```");
         $this->assertHtmlGivesMarkdown("<pre>foo\n</pre>", "```\nfoo\n```");
-        $this->assertHtmlGivesMarkdown("<pre>\nfoo</pre>", "```\n\nfoo\n```");
-        $this->assertHtmlGivesMarkdown("<pre>\nfoo\n</pre>", "```\n\nfoo\n```");
-        $this->assertHtmlGivesMarkdown("<pre>\n\n</pre>", "```\n\n\n```");
-        $this->assertHtmlGivesMarkdown("<pre>\n\n\n</pre>", "```\n\n\n\n```");
-        $this->assertHtmlGivesMarkdown("<pre>\n</pre><pre>\n</pre>", "```\n\n```\n\n```\n\n```");
+        $this->assertHtmlGivesMarkdown("<pre>\nfoo</pre>", "```\nfoo\n```");
+        $this->assertHtmlGivesMarkdown("<pre>\nfoo\n</pre>", "```\nfoo\n```");
+        $this->assertHtmlGivesMarkdown("<pre>\r\nfoo</pre>", "```\nfoo\n```");
+        $this->assertHtmlGivesMarkdown("<pre>\n\nfoo</pre>", "```\n\nfoo\n```");
+        $this->assertHtmlGivesMarkdown("<pre>\n\n</pre>", "```\n\n```");
+        $this->assertHtmlGivesMarkdown("<pre>\n\n\n</pre>", "```\n\n\n```");
+        $this->assertHtmlGivesMarkdown("<pre>\n</pre><pre>\n</pre>", "```\n```\n\n```\n```");
         $this->assertHtmlGivesMarkdown("<pre>one\ntwo\r\nthree</pre>\n<p>line</p>", "```\none\ntwo\nthree\n```\n\nline");
         $this->assertHtmlGivesMarkdown("<pre class='some-class'>test with attributes</pre>", "```\ntest with attributes\n```");
     }