Reset active context to initial context if set to null

Updated the context processing algorithm. Depending of the outcome of issue #80 this might has to be changed.

This closes #78.

Author: lanthaler

spec/latest/json-ld-api/index.html

@@ -821,6 +821,7 @@ <h2 id="context">Context</h2>
     <li>Create a new, empty <tref>local context</tref>.</li>
     <li>Let <em>value</em> be the value of <code>@context</code>
       <ol id="process-context" class="algorithm">
+        <li>If <em>value</em> equals <code>null</code>, reset the <tref>active context</tref> to the <tref>initial context</tref></li>
         <li>If <em>value</em> is an <tref>array</tref>, process
           each element as <em>value</em>, in order using <a href="#process-context">Step 2</a>.</li>
         <li>If <em>value</em> is a simple <tref>string</tref>, it MUST have a lexical form of <tref>absolute IRI</tref>.
@@ -873,7 +874,7 @@ <h2 id="context">Context</h2>
                   </ol>
                 </li>
                 <li>Merge the <tref>local context</tref> into the <tref>active context</tref>.</li>
-                <li>Repeat <a href="#object-context">Step 2.3.2</a> until no entries are added to the <tref>local
+                <li>Repeat <a href="#object-context">Step 3.3.2</a> until no entries are added to the <tref>local
                   context</tref>.</li>
               </ol>
             </li>