clarified that content-type can be used with all frames (issue #53)

src/stomp-specification-1.2.md

@@ -75,7 +75,7 @@ some areas of the specification such as:
 
 * repeated frame header entries
 
-* use of the `content-type` header
+* use of the `content-length` and `content-type` headers
 
 * required support of the `STOMP` frame by servers
 
@@ -180,13 +180,15 @@ Some headers MAY be used, and have special meaning, with most frames.
 
 #### Header content-length
 
-The `SEND`, `MESSAGE` and `ERROR` frames SHOULD include a `content-length`
-header if a frame body is present. If a frame's body contains NULL octets, the
-frame MUST include a `content-length` header. The header is an octet count for
-the length of the message body. If a `content-length` header is included, this
-number of octets MUST be read, regardless of whether or not there are NULL
-octets in the body. The frame still needs to be terminated with a NULL
-octet.
+All frames MAY include a `content-length` header. This header is an octet
+count for the length of the message body. If a `content-length` header is
+included, this number of octets MUST be read, regardless of whether or not
+there are NULL octets in the body. The frame still needs to be terminated
+with a NULL octet.
+
+If a frame body is present, the `SEND`, `MESSAGE` and `ERROR` frames SHOULD
+include a `content-length` header to ease frame parsing. If the frame body
+contains NULL octets, the frame MUST include a `content-length` header.
 
 #### Header content-type