fix bug of  post 'multipart/form-data' request without file data
Merge pull request #226 from coffeesherk/master

fix bug of post 'multipart/form-data' request without file data
add content-type parameter for char-stream data in multipart/form-data

add content-type parameter for char-stream data in multipart/form-data &
fix bug of writing wrong last boundary
Merge pull request #228 from coffeesherk/master

add content-type parameter for char-stream data in multipart/form-data & fix bug of writing wrong last boundary