Take raw pointer and not wxSharedPtr<> in SetData()

Using shared pointer seems to be ill-advised here, the stream shouldn't
be shared as it's going to be used by wxWebRequest itself and can't be
used by the application code in parallel, so the ownership transfer
semantics is more appropriate.

We could take a wxScopedPtr<> instead, but wx API takes ownership of raw
pointers everywhere else, so do it here too.

Incidentally fix a bug with calling IsOk() on a possibly null pointer.

interface/wx/webrequest.h

@@ -269,10 +269,23 @@ public:
         @c GET and the given @a dataStream will be posted as the body of
         this request.
 
+        Example of use:
+        @code
+        std::unique_ptr<wxInputStream> stream(new wxFileInputStream("some_file.dat"));
+        if ( !stream->IsOk() ) {
+            // Handle error (due to e.g. file not found) here.
+            ...
+            return;
+        }
+        request.SetData(stream.release(), "application/octet-stream")
+        @endcode
+
         @param dataStream
             The data in this stream will be posted as the request body. The
-            stream may be empty, which will result in sending 0 bytes of data,
-            but if not empty, should be valid.
+            pointer may be @NULL, which will result in sending 0 bytes of data,
+            but if not empty, should be valid, i.e. wxInputStream::IsOk() must
+            return @true. This object takes ownership of the passed in pointer
+            and will delete it, i.e. the pointer must be heap-allocated.
         @param contentType
             The value of HTTP "Content-Type" header, e.g.
             "application/octet-stream".
@@ -284,7 +297,7 @@ public:
             dataSize is not specified and the attempt to determine stream size
             failed; @true in all the other cases.
     */
-    bool SetData(const wxSharedPtr<wxInputStream>& dataStream,
+    bool SetData(wxInputStream* dataStream,
         const wxString& contentType, wxFileOffset dataSize = wxInvalidOffset);
 
     /**