Fix HttpWebRequest.IfModifiedSince (#3854)

* Fix HttpWebRequest.IfModifiedSince

* Apply suggestions from code review

Co-Authored-By: Maira Wenzel <[email protected]>

Co-authored-by: Maira Wenzel <[email protected]>

Author: nxtn

xml/System.Net/HttpWebRequest.xml

@@ -3266,12 +3266,14 @@ The GetHashCode method returns a hash code of the web request. This value can be
         <ReturnType>System.DateTime</ReturnType>
       </ReturnValue>
       <Docs>
-        <summary>Gets or sets the value of the <see langword="If-Modified-Since" /> HTTP header.</summary>
-        <value>A <see cref="T:System.DateTime" /> that contains the contents of the <see langword="If-Modified-Since" /> HTTP header. The default value is the current date and time.</value>
+        <summary>Gets or sets the value of the <c>If-Modified-Since</c> HTTP header.</summary>
+        <value>A <see cref="T:System.DateTime" /> that contains the contents of the <see langword="If-Modified-Since" /> HTTP header.</value>
         <remarks>
           <format type="text/markdown"><![CDATA[  
   
 ## Remarks  
+ If the `If-Modified-Since` header is `null`, then the return value will be set to <xref:System.DateTime.MinValue?displayProperty=nameWithType>.  
+  
  The <xref:System.Net.HttpWebRequest.IfModifiedSince%2A> property is a standard <xref:System.DateTime?displayProperty=nameWithType> object and can contain a <xref:System.DateTimeKind?displayProperty=nameWithType> field of <xref:System.DateTimeKind.Local?displayProperty=nameWithType>, <xref:System.DateTimeKind.Utc?displayProperty=nameWithType>, or <xref:System.DateTimeKind.Unspecified?displayProperty=nameWithType>. Any kind of time can be set when using the <xref:System.Net.HttpWebRequest.IfModifiedSince%2A> property. If <xref:System.DateTimeKind.Unspecified?displayProperty=nameWithType> is set or retrieved, the <xref:System.Net.HttpWebRequest.IfModifiedSince%2A> property is assumed to be <xref:System.DateTimeKind.Local?displayProperty=nameWithType> (local time).  
   
  The classes in the <xref:System.Net> namespace always write it out the <xref:System.Net.HttpWebRequest.IfModifiedSince%2A> property on the wire during transmission in standard form using GMT (Utc) format.  
@@ -3283,6 +3285,8 @@ The GetHashCode method returns a hash code of the web request. This value can be
 > [!NOTE]
 >  The value for this property is stored in <xref:System.Net.WebHeaderCollection>. If WebHeaderCollection is set, the property value is lost.  
   
+ If the <xref:System.Net.HttpWebRequest.IfModifiedSince%2A> property is set and 304 (Not Modified) status code is returned, then a <xref:System.Net.WebException?displayProperty=nameWithType> will be thrown by the <xref:System.Net.HttpWebRequest.BeginGetResponse%2A>, <xref:System.Net.HttpWebRequest.GetResponse%2A>, and <xref:System.Net.HttpWebRequest.EndGetResponse%2A> methods.  
+  
    
   
 ## Examples