Move Pad High and Pad Low to right after frame header for HEADERS

Padding is made to a frame and other frames
(DATA and CONTINUATION) have these 2 bytes right after the frame
header. It is easier for implementation to put the same constructs
in the same position.
Merge pull request #380 from tatsuhiro-t/move-pad-high-low-right-after-frame-header

Move Pad High and Pad Low to right after frame header for HEADERS