title: issue while trying to implement node js multimap
body: <p>I am trying to use Multimap as I may have different values for same key. However I don't want entry into it when Key-value pair is already present in Multimap. 
  But unfortunately, when I use MultiMap it is creating new entries for same key-value pair in multimap. Please let me know if I am missing something ?? code snippet is given below -</p>
  
  <pre><code>//declared MultiMap -
  var Multimap = require('multimap');
  var mmap = new Multimap();
  </code></pre>
  
  <p>//ran command on console to include multimap to project: </p>
  
  <pre><code>npm install multimap --save
  </code></pre>
  
  <p>//pushed data to multimap using set method</p>
  
  <pre><code>mmap.set(path, columns[1]['value']);
  </code></pre>
  
  <p>//checked for size of multimap</p>
  
  <pre><code>mmap.length
  </code></pre>
  
  <p>//printed each entry in multimap</p>
  
  <pre><code>mmap.forEachEntry(function (entry, key) {
  console.log(key + "----" + entry);
  });
  </code></pre>
  
  <p>//OUTPUT for key----value in multimap</p>
  
  <pre><code>product_tree,SBG,QuickBooks,39.0,R2----R2
  product_tree,SBG,QuickBooks,39.0,R1----R1
  product_tree,SBG,QuickBooks,28.0----28.0,28.0,28.0,28.0,28.0,28.0,28.0
  product_tree,SBG,QuickBooks,28.0,R1----R1,R1
  product_tree,SBG,QuickBooks,28.0,R7----R7
  product_tree,SBG,QuickBooks,28.0,R6----R6
  product_tree,SBG,QuickBooks,28.0,R5----R5
  product_tree,SBG,QuickBooks,28.0,R4----R4
  product_tree,SBG,QuickBooks,28.0,R3----R3
  product_tree,SBG,QuickBooks,27.0----27.0,27.0,27.0,27.0,27.0,27.0,27.0,27.0,27.0,27.0
  product_tree,SBG,QuickBooks,27.0,R1----R1,R1
  </code></pre>
  
  <p>//expected output for key----value</p>
  
  <pre><code>product_tree,SBG,QuickBooks,39.0,R2----R2
  product_tree,SBG,QuickBooks,39.0,R1----R1
  product_tree,SBG,QuickBooks,28.0----28.0
  product_tree,SBG,QuickBooks,28.0,R1----R1
  product_tree,SBG,QuickBooks,28.0,R7----R7
  product_tree,SBG,QuickBooks,28.0,R6----R6
  product_tree,SBG,QuickBooks,28.0,R5----R5
  product_tree,SBG,QuickBooks,28.0,R4----R4
  product_tree,SBG,QuickBooks,28.0,R3----R3
  product_tree,SBG,QuickBooks,27.0----27.0
  product_tree,SBG,QuickBooks,27.0,R1----R1
  </code></pre>
  
  <p>Would appreciate if I get pointer about where am I doing mistake. </p>
  
  <p>Thanks in advance.</p>