Description:
------------
It seems that the float to string conversion is not working correctly for SolrInputDocument::setBoost() when the PHP locale contains the rule, that the delimiter is something else than a dot. The value will be written with the formatting defined by the locale in the XML file, which leads to a 500 error on Solr server side.



Reproduce code:
---------------
// With German PHP locale
setlocale(LC_ALL, 'de_DE');
echo (string) 1.5; // will output 1,5

// and
$doc = new SolrInputDocument();
$doc->setBoost(2.5);
...

results in the following XML:
  <doc boost="2,5">
    <field name="id">177683</field>


and Solr responds:

HTTP/1.1 500 Internal Server Error
Server: Apache-Coyote/1.1
...
HTTP Status 500 - For input string: &quot;2,5&quot;



Expected result:
----------------
<doc boost="2.5">

Actual result:
--------------
<doc boost="2,5">