improvements/changes

Hi,
after using the apache module mapCache, I had to do some changes to the code to fit best my needs.

I would like to share my considerations on some behavior: 

1) when you request a tile outside the extension of the tileset, you get the error.
with the configuration of the errors as empty_img it returns an empty image but the http response code was annoying, so I changed it to a response code of 200 and the browser will not log all the errors. 

2) configuring the mapcache with a jpeg output format, when I request a tile that is half-in and half-out the extension of the tileset, the half-out image had a black background, as I use maps with a white background, I changed it using the alpha channel
to determine which part to change to white. 

3) requesting images as png to maintain transparency, I had some trouble with the default image format which was JPEG,
this because in the code, the default format is get by dont know what and by default was jpeg.
I suggest to change it to take the default format specified in the mapcache xml configuration. 

I would like to know what you guys think of my changes.