Improve JSON output when there is leading data before the actual JSON body (#1130)

In some special cases, to prevent against Cross Site Script Inclusion (XSSI)
attacks, the JSON response body starts with a magic prefix line that must be
stripped before feeding the rest of the response body to the JSON parser.
Such prefix is now simply ignored from the parser but still printed in the
terminal.

* Fix Windows tests

tests/test_json.py

@@ -0,0 +1,40 @@
+import json
+
+import pytest
+import responses
+
+from httpie.cli.constants import PRETTY_MAP
+from httpie.compat import is_windows
+from httpie.output.formatters.colors import ColorFormatter
+
+from .utils import MockEnvironment, http, URL_EXAMPLE
+
+TEST_JSON_XXSI_PREFIXES = (r")]}',\n", ")]}',", 'while(1);', 'for(;;)', ')', ']', '}')
+TEST_JSON_VALUES = ({}, {'a': 0, 'b': 0}, [], ['a', 'b'], 'foo', True, False, None)  # FIX: missing int & float
+TEST_PREFIX_TOKEN_COLOR = '\x1b[38;5;15m' if is_windows else '\x1b[04m\x1b[91m'
+
+
+@pytest.mark.parametrize('data_prefix', TEST_JSON_XXSI_PREFIXES)
+@pytest.mark.parametrize('json_data', TEST_JSON_VALUES)
+@pytest.mark.parametrize('pretty', PRETTY_MAP.keys())
+@responses.activate
+def test_json_formatter_with_body_preceded_by_non_json_data(data_prefix, json_data, pretty):
+    """Test JSON bodies preceded by non-JSON data."""
+    body = data_prefix + json.dumps(json_data)
+    content_type = 'application/json'
+    responses.add(responses.GET, URL_EXAMPLE, body=body,
+                  content_type=content_type)
+
+    colored_output = pretty in ('all', 'colors')
+    env = MockEnvironment(colors=256) if colored_output else None
+    r = http('--pretty=' + pretty, URL_EXAMPLE, env=env)
+
+    indent = None if pretty in ('none', 'colors') else 4
+    expected_body = data_prefix + json.dumps(json_data, indent=indent)
+    if colored_output:
+        fmt = ColorFormatter(env, format_options={'json': {'format': True, 'indent': 4}})
+        expected_body = fmt.format_body(expected_body, content_type)
+        # Check to ensure the non-JSON data prefix is colored only one time,
+        # meaning it was correctly handled as a whole.
+        assert TEST_PREFIX_TOKEN_COLOR + data_prefix in expected_body, expected_body
+    assert expected_body in r