Encoding floats in a dictionary using custom python JSON encoder

Here's a simple JSON encoder that convert float values to string:

class NestedEncoder(json.JSONEncoder):
    '''
    A JSON Encoder that converts floats/decimals to strings and allows nested objects
    '''

    def default(self, obj):

        if isinstance(obj, float) or obj.__class__.__name__ == "float32":
            return self.floattostr(obj)
        elif obj.__class__.__name__ == "type":
            return str(obj)
        elif hasattr(obj, 'repr_json'):
            return obj.repr_json()
        else:
            return json.JSONEncoder.default(self, obj)

    def floattostr(self,o,_inf=float('Inf'), _neginf=-float('-Inf'),nan_str="None"):
        if o != o:
            text = nan_str
        elif o == _inf:
            text = 'Infinity'
        elif o == _neginf:
            text = '-Infinity'
        else:
            return o.__repr__()

        return text

Now here's two tests. The first creates a Infinity valued float and encodes it using the custom encoder. Test passes.

def test_inf():
    inf = float('Inf')
    as_json = json.dumps(inf,cls=NestedEncoder)
    assert as_json == "Infinity"

The second test does the same but puts the float in a dictionary:

def test_inf_dic():
    inf = float('Inf')
    as_json = json.dumps({'key':inf},cls=NestedEncoder)
    assert as_json == "{'key':'Infinity'}"

OUTPUT:

=================================== FAILURES ===================================
_________________________________ test_inf_dic _________________________________

    def test_inf_dic():
        inf = float('Inf')
        as_json = json.dumps({'key':inf},cls=NestedEncoder)
>       assert as_json == "{'key':'Infinity'}"
E       assert '{"key": Infinity}' == "{'key':'Infinity'}"
E         - {"key": Infinity}
E         ?  ^   ^ ^
E         + {'key':'Infinity'}
E         ?  ^   ^ ^        +

EDIT:

The custom encoder only gets called for the first test, not the second.