Fix iter_lines boundary bug when CRLF straddles two chunks

[nanotone]

In Response.iter_lines, if a \r\n sequence (the only multi-character line boundary) straddles two chunks, the \r and \n are treated as two separate boundaries. Consequently, iter_lines will yield one extra blank line, as an empty string.

This addresses the issue by checking if a chunk ends with \r, and when appropriate, leaving the trailing line in pending until the next chunk or EOF arrives.

[nateprewitt]

Hi @nanotone, thanks for putting this together! This has actually already been addressed and is patched for Requests-3.0 (#3984). We won't be able to merge this into the current master branch because it's a breaking change for some users which isn't backwards compatible.