minor #4345 Correct capitalization for the Content-Type header (GeertDD)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #4345).

Discussion
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Correct capitalization for the Content-Type header

http://tools.ietf.org/html/rfc7231#section-3.1.1.5

Commits
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74dc6f8 Correct capitalization for the Content-Type header

book/http_fundamentals.rst

@@ -186,7 +186,7 @@ PHP? In reality, PHP abstracts you a bit from the whole process::
     $uri = $_SERVER['REQUEST_URI'];
     $foo = $_GET['foo'];
 
-    header('Content-type: text/html');
+    header('Content-Type: text/html');
     echo 'The URI requested is: '.$uri;
     echo 'The value of the "foo" parameter is: '.$foo;