RateLimitReachedError exception sets message, code and headers.
RateLimitReachedError exception sets message, code and headers.

Added docstring documentation describing HTTP-date format
translation on Retry-After header.

Signed-off-by: Quentin Pradet <d17611fde61d73f8de1b8b01bc47d446558ccd8e@apache.org>

andrewsomething/libcloud,pquentin/libcloud,Kami/libcloud,vongazman/libcloud,apache/libcloud,andrewsomething/libcloud,mistio/libcloud,vongazman/libcloud,mistio/libcloud,Scalr/libcloud,StackPointCloud/libcloud,ByteInternet/libcloud,apache/libcloud,Scalr/libcloud,apache/libcloud,andrewsomething/libcloud,Scalr/libcloud,Kami/libcloud,StackPointCloud/libcloud,mistio/libcloud,Kami/libcloud,pquentin/libcloud,pquentin/libcloud,vongazman/libcloud,ByteInternet/libcloud,ByteInternet/libcloud,StackPointCloud/libcloud