/$query shall allow Content-Type: application/x-www-form-urlencoded

An HTML form can be used to have a browser navigate to the response of an OData request, especially if the response is a stream. If that OData request makes use of the /$query segment, it must have Content-Type: text/plain, which forces an HTML form like

<form method="post" action="~/$query" enctype="text/plain">
 <input name="$filter" value="a%20eq%201&$select=b"/>
</form>

Much more natural would be if Content-Type: application/x-www-form-urlencoded were also allowed:

<form method="post" action="~/$query"
  enctype="application/x-www-form-urlencoded">
 <input name="$filter" value="a eq 1"/>
 <input name="$select" value="b"/>
</form>

This leads to the same request body as above:

$filter=a%20eq%201&$select=b

but without forcing the client to percent-encode spaces and insert the ampersand.