fix: skip schema lookup when value is not a str (#401)

Since 84674909 (#351, "added binary support in dictionaries via base64
encoding"), `_DynamicStructBuilder.from_dict` does a
`self.schema.fields.get(key)` lookup for every key in the input dict.
The lookup was added so that `str` values destined for a `Data` field
can be base64-decoded, but the lookup itself runs unconditionally, even
when the value is `bool`, `int`, `dict`, `list`, `bytes`, etc.

capnp/lib/capnp.pyx

@@ -1738,8 +1738,8 @@ cdef class _DynamicStructBuilder:
     def from_dict(self, dict d):
         for key, val in d.iteritems():
             if key != 'which':
-                field = self.schema.fields.get(key)
                 if isinstance(val, str):
+                    field = self.schema.fields.get(key)
                     dtype = field.proto.slot.type.which()
                     if dtype == "data":
                         # decode bytes from utf-8 base64 encoding