minor #10651 Rename content_type header to content-type (ottaviano)

This PR was submitted for the master branch but it was merged into the 2.8 branch instead (closes #10651).

Discussion
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Rename content_type header to content-type

Commits
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9feaac0 Rename content_type header to content-type

Author: xabbuh

controller.rst

@@ -487,7 +487,7 @@ the ``Request`` class::
 
         // retrieves an HTTP request header, with normalized, lowercase keys
         $request->headers->get('host');
-        $request->headers->get('content_type');
+        $request->headers->get('content-type');
     }
 
 The ``Request`` class has several public properties and methods that return any

create_framework/http_foundation.rst

@@ -192,7 +192,7 @@ fingertips thanks to a nice and simple API::
 
     // retrieve an HTTP request header, with normalized, lowercase keys
     $request->headers->get('host');
-    $request->headers->get('content_type');
+    $request->headers->get('content-type');
 
     $request->getMethod();    // GET, POST, PUT, DELETE, HEAD
     $request->getLanguages(); // an array of languages the client accepts

introduction/http_fundamentals.rst

@@ -231,7 +231,7 @@ have all the request information at your fingertips::
 
     // retrieves an HTTP request header, with normalized, lowercase keys
     $request->headers->get('host');
-    $request->headers->get('content_type');
+    $request->headers->get('content-type');
 
     $request->getMethod();    // e.g. GET, POST, PUT, DELETE or HEAD
     $request->getLanguages(); // an array of languages the client accepts