Bugfix #373 the get() operation not accepting multiple subtree filters

[fredgan]

Hi,
This PR fixes issue #373, It allows the build_filter()'s spec parameter to be a list of filters

my testcase is like that:

# a list of subtree filters is valid for subtree filters
FILTER=[
      "<cont xmlns=\"urn:mod1\"> \
            <le1/> \
            <le2/> \
       </cont>",
      "<cont xmlns=\"urn:mod2\">"
]
  
def connect(host, port, user, password, source):
    conn = manager.connect(host=host,
                           port=port,
                           username=user,
                           password = password,
                           device_params={'name': 'iosxr'}
                           )

    rpc_reply = conn.get(filter=FILTER).xml
    print(rpc_reply)

if __name__ == "__main__":
    connect(host, port, user, password)

The output is that:

<nc:rpc-reply xmlns:nc="urn:ietf:params:xml:ns:netconf:base:1.0" message-id="urn:uuid:1ebdec8f-4aba-4ceb-b076-20f45cb7fde5"><data xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"><cont xmlns="urn:mod1"><le1>this is mod1</le1><le2>111</le2></cont><cont xmlns="urn:mod2"><le1>this is mod2</le1><le2>222</le2><lst><le3>222222222</le3></lst></cont></data></nc:rpc-reply>

[coveralls]

Coverage increased (+0.05%) to 68.339% when pulling 8fc380c on fredgan:multiple_subtree_filters into 790c1e9 on ncclient:master.

[einarnn]

Thanks! And sorry for the slow review!