timezone lost in <time> fields when cloning/copying parsed GPX file

[3add3287]

When parsing a GPX file that doesn't use UTC but TZ aware timestamps, the timestamp is lost when using the clone method. The cloned gpx object now shows the same time as before, but with the wrong timezone. "2018-04-04T10:36:53-0600" now becomes "2018-04-04T10:36:53Z"

The following illustrates the problem:

#!/usr/local/bin/python3.7
import sys
import re

import gpxpy

with open(sys.argv[1], mode='r') as gpx_file:
    GPX = gpxpy.parse(gpx_file)
    GPX_COPY = GPX.clone()

    GPX_XML_ORIG = GPX.to_xml()
    GPX_XML_COPY = GPX_COPY.to_xml()

    RETIME = re.compile(r'<time>.*</time>')
    TIMES_ORIG = RETIME.findall(GPX_XML_ORIG)
    TIMES_COPY = RETIME.findall(GPX_XML_COPY)

    print("Original:\n{}\n\nCopy:\n{}\n".format(
        "\n".join(TIMES_ORIG[1:5]),
        "\n".join(TIMES_COPY[1:5])
    ))

Using a GPX file with TZ aware timestamps:

./test-gpxpy-tz.py GarminNuviTest.gpx
Original:
<time>2018-04-04T10:36:53-0600</time>
<time>2018-04-04T10:41:07-0600</time>
<time>2018-04-04T10:44:44-0600</time>
<time>2018-04-04T10:44:50-0600</time>

Copy:
<time>2018-04-04T10:36:53Z</time>
<time>2018-04-04T10:41:07Z</time>
<time>2018-04-04T10:44:44Z</time>
<time>2018-04-04T10:44:50Z</time>

python 3.7.6 and Gpxpy 1.3.5