Warn when using --ignore-stdin with piped input #1525

[blanca-negrete]

This PR adds a warning when the --ignore-stdin flag is used while there is data coming through stdin. This prevents confusion about why piped input is not being processed.

Modified httpie/cli/argparser.py to check if stdin has content when --ignore-stdin is set. Raises a UserWarning to alert the user that the input will be ignored.