Allow surrogates in content, issue 895

Author: mihnita

spec/message.abnf

@@ -76,8 +76,7 @@ content-char      = %x01-08        ; omit NULL (%x00), HTAB (%x09) and LF (%x0A)
                   / %x41-5B        ; omit \ (%x5C)
                   / %x5D-7A        ; omit { | } (%x7B-7D)
                   / %x7E-2FFF      ; omit IDEOGRAPHIC SPACE (%x3000)
-                  / %x3001-D7FF    ; omit surrogates
-                  / %xE000-10FFFF
+                  / %x3001-10FFFF  ; allowing surrogates is intentional
 
 ; Character escapes
 escaped-char = backslash ( backslash / "{" / "|" / "}" )