Fix wxTranslations::GetHeaderValue() to look for correct header

Correct the search to completely match the full header name and not only
its beginning by appending ": " to the specified search string.

This prevents GetHeaderValue("Language") from returning "eam: wxWidgets team"
if there is a "Language-Team" header in the catalog, for example.

Closes #17555.

src/common/translation.cpp

@@ -1721,7 +1721,7 @@ wxString wxTranslations::GetHeaderValue(const wxString& header,
     if ( !trans || trans->empty() )
         return wxEmptyString;
 
-    size_t found = trans->find(header);
+    size_t found = trans->find(header + wxS(": "));
     if ( found == wxString::npos )
         return wxEmptyString;