title: Remove null params in HttpParams
body: <p>I have a criteria object for which some attributes can be null, if I don't do anything the query string include those like <code>&amp;p=undefined</code> which is not desirable since at the WebApi these come as<code>"null"</code> instead of <code>null</code> 
  So given this in the Angular client </p>
  
  <pre><code>      return this._http
            .get&lt;ExposureDifference[]&gt;(AppSettings.ExposureRunDataQueryString, {params : &lt;any&gt;criteria, withCredentials: true}) 
            .catch&lt;any, ExposureDifference[]&gt;(this._trace.handleError("GET " + AppSettings.ExposureRunDataQueryString + criteria, []))
            .finally(() =&gt; this.isLoading = false);
  </code></pre>
  
  <p>I could get the query string as </p>
  
  <pre><code>http://localhost:63037/api/exposureRuns/data/?id=3&amp;secsymb=undefined&amp;fund=EL&amp;esectype=SWAP_CDS
  </code></pre>
  
  <p>Is there a way to exclude the undefined parameters from the query string?</p>