fix bug(Content-Length becomes zero)

The return value of MagickGetImageFormat is NULL
when using parameters of 'cw' and 'ch'.
fix bug(invalid paramenter pass through)

when the parameters of 'cw' and 'ch' is not used and 'bw' and 'bh' is used,
border size becomes strange.
support GD graphics library
v0.3.0 released