Fix some warnings

lib/rdf/inspect.ex

@@ -94,7 +94,6 @@ end
 
 defimpl Inspect, for: RDF.Dataset do
   import Inspect.Algebra
-  import RDF.InspectHelper
 
   def inspect(%RDF.Dataset{name: name} = dataset, opts) do
     doc =

lib/rdf/serializations/nquads_encoder.ex

@@ -3,7 +3,7 @@ defmodule RDF.NQuads.Encoder do
 
   use RDF.Serialization.Encoder
 
-  def encode(data, opts \\ []) do
+  def encode(data, _opts \\ []) do
     result =
       data
       |> Enum.reduce([], fn (statement, result) ->

lib/rdf/serializations/ntriples_encoder.ex

@@ -7,7 +7,7 @@ defmodule RDF.NTriples.Encoder do
 
   @xsd_string RDF.Datatype.NS.XSD.string
 
-  def encode(data, opts \\ []) do
+  def encode(data, _opts \\ []) do
     result =
       data
       |> Enum.reduce([], fn (statement, result) ->

lib/rdf/serializations/turtle_encoder.ex

@@ -290,7 +290,7 @@ defmodule RDF.Turtle.Encoder do
     end
   end
 
-  defp term(%Literal{datatype: datatype} = literal, state, _, nesting),
+  defp term(%Literal{} = literal, state, _, nesting),
     do: typed_literal_term(literal, state, nesting)
 
   defp term(list, state, _, nesting) when is_list(list) do