Fix type of value in error middleware response header
Fix type of value in error middleware response header

Apache mod_wsgi is strict about WSGI specification violations,
all response headers must be strings. This patch fixes value
of 'Content-Length' header.

Closes-Bug: #1441014
Change-Id: Iab82cac883efb19886527c35a2cb0055cbe0a6ce

pshchelo/ironic,naototty/vagrant-lxc-ironic,SauloAislan/ironic,devananda/ironic,NaohiroTamura/ironic,naterh/ironic,ionutbalutoiu/ironic,SauloAislan/ironic,rdo-management/ironic,ionutbalutoiu/ironic,pshchelo/ironic,supermari0/ironic,dims/ironic,redhat-openstack/ironic,hpproliant/ironic,bacaldwell/ironic,openstack/ironic,NaohiroTamura/ironic,dims/ironic,bacaldwell/ironic,naototty/vagrant-lxc-ironic,openstack/ironic,Tan0/ironic