Error method parsed when more data append request buffer

I made a demo
```
try:
from http_parser.parser import HttpParser
except ImportError:
from http_parser.pyparser import HttpParser

req = "GET / HTTP/1.1\r\nHost: www.xxx.com\r\n\r\n"
resp = "HTTP/1.1 200 OK\r\nContent-Length: 4\r\n\r\n1234"

data = req
p = HttpParser()
nparsed = p.execute(data, len(data))
print p.get_method(), nparsed, len(data)
# Got: GET 37 37

data = req + resp
p = HttpParser()
nparsed = p.execute(data, len(data))
print p.get_method(), nparsed, len(data)
# Got: HEAD 38 79
# pyparser Got: GET 79 79

# expected: GET 37 79
```