[nugets_pack] fetch `IconUrl` element from fsproj

[haf]

Doesn't seem to do so: SuaveIO/suave@691dbed

Output:

<?xml version="1.0"?>
<package xmlns="http://schemas.microsoft.com/packaging/2011/08/nuspec.xsd">
  <metadata>
    <id>Suave.LibUv</id>
    <version>0.33.0</version>
    <title>Suave.LibUv</title>
    <authors>Ademar Gonzalez, Henrik Feldt</authors>
    <owners>Ademar Gonzalez, Henrik Feldt</owners>
    <licenseUrl>https://github.com/SuaveIO/Suave/blob/master/COPYING</licenseUrl>
    <projectUrl>http://suave.io</projectUrl>
    <iconUrl>https://raw.githubusercontent.com/SuaveIO/resources/master/images/head_trans.png</iconUrl>
    <requireLicenseAcceptance>false</requireLicenseAcceptance>
    <description>Suave is a simple web development F# library providing a lightweight web server and a set of combinators to manipulate route flow and task composition.</description>
    <releaseNotes>Release Notes:

snip
 * updated prpject file
 * libuv wip</releaseNotes>
    <copyright>Ademar Gonzalez, Henrik Feldt</copyright>
    <language>en-GB</language>
    <dependencies>
      <dependency id="FSharp.Core" version="3.1.2.5" />
      <dependency id="Suave" version="0.33.0" />
    </dependencies>
  </metadata>
</package>