Cast header Lambda-Runtime-Deadline-Ms to int for Python2.7/3.6 runtimes

[jhaenchen]

We are experiencing an issue with 2.7 runtime when calling get_remaining_time_in_millis. It appears that in the runtime the variable DEADLINE_MS is overwritten with a value pulled directly from a header, which is a string. Then when get_remaining_time_in_millis is called, it tries to subtract an int from a string, and so throws. This PR simply casts the received header value to an int before overwriting the variable.

Proof of bug:

"""
Save this to a file named lambda_function.py

Runs without problem in python 3.7
docker run --rm -v $PWD:/var/task lambci/lambda:python3.7 lambda_function.lambda_handler

Has problems in python 2.7 and 3.6
docker run --rm -v $PWD:/var/task lambci/lambda:python2.7 lambda_function.lambda_handler
docker run --rm -v $PWD:/var/task lambci/lambda:python3.6 lambda_function.lambda_handler
"""

def lambda_handler(event, context):
    # https://docs.aws.amazon.com/lambda/latest/dg/python-context-object.html
    func_name = context.function_name
    rem_time_ms = context.get_remaining_time_in_millis()

    return {
        "func_name": func_name,
        "rem_time_ms": rem_time_ms,
    }