Merge pull request nexpy#252 from rayosborn:check-NXfield-arguments

Check NXfield arguments

Author: rayosborn

src/nexusformat/definitions/base_classes/NXentry.nxdl.xml

@@ -208,7 +208,7 @@
 			A small image that is representative of the entry. An example of this is a 640x480
 			jpeg image automatically produced by a low resolution plot of the NXdata.
 		</doc>
-		<attribute name="type">
+		<attribute name="type" deprecated="Use the `type` field instead">
 			<doc>The mime type should be an ``image/*``</doc>
 			<enumeration>
 				<!--

src/nexusformat/definitions/base_classes/NXinstrument.nxdl.xml

@@ -60,6 +60,7 @@
     <group type="NXfermi_chopper" />
     <group type="NXfilter" />
     <group type="NXflipper" />
+    <group type="NXgoniometer" />
     <group type="NXguide" />
     <group type="NXhistory"/>
     <group type="NXinsertion_device" />

src/nexusformat/nexus/tree.py

@@ -2912,7 +2912,7 @@ def __init__(self, value=None, name='unknown', shape=None, dtype=None,
                             if v is not None)
         if attrs is None:
             attrs = {}
-        attrs.update(kwargs)
+        attrs.update({k: v for k, v in kwargs.items() if v is not None})
         self._attrs = AttrDict(self, attrs=attrs)
         self._memfile = None
         self._uncopied_data = None