fix issue #568 - strict conneg should return 406 if no acceptable variant

Use case:

Client does a conditional GET with an Accept header specifying a media
type not advertised by the target resource (via its @get annotation).

The current state of the resource satisfies the condition.

The server has requested strict content negotation

Current behavior: ServerResource.doConditionalHandle() gets a null Variant
back from getPreferredVariant() and passes this to
doGetInfo(Variant target), which calls doGetInfo(), i.e. the variation
with no params. If any registered converter is able to produce a
representation for a null target Variant (e.g. GwtConverter), then
whichever one of those converters "wins" will produce a representation that
gets returned back to doConditionalHandle. Later in that method, the
following code is executed:

if ((Method.GET.equals(​getMethod()) || Method.HEAD
                .equals(getMethod()))
                && resultInfo instanceof Representation) {
            result = (Representation) resultInfo;

and since the if() statement is true, it proceeds to return the result, and
the client gets back the converter-generated representation instead of a
406.

The fix: in doConditionalHandle(), change

if (existing) {
    if (isNegotiated()) {
            resultInfo = doGetInfo(getPreferredVariant(getVariants(Method.GET)));
    }
to

if (existing) {
    if (isNegotiated()) {
        Variant preferredVariant = getPreferredVariant(getVariants(Method.GET));
        if (preferredVariant == null
                && getConnegService().isStrict()) {
            doError(Status.CLIENT_ERROR_NOT_ACCEPTABLE);
        } else {
            resultInfo = doGetInfo(preferredVariant);
        }
- Fixed lack of warnings when entity reading/writing issues occur with internal connector.
- Fixed BioUtils#getInputStream to close pipe in case of exception
 - Renamed Directory#setAlphaComparator and setAlphaNumComparator
   methods into useAlphaComparator and useAlphaNumComparator with
   proper deprecation. Suggested by Tim Peierls.
Merge branch 'master' of github.com:restlet/restlet-framework-java
Fixed Client context parameters (issue #606)
Merge pull request #607 from alepac/patch-1

Fixed Client context parameters (issue #606)
Merge branch 'master' of github.com:restlet/restlet-framework-java
       - Fixed concurrency bug upon application start or strop that could
         cause erros such as "no Restlet defined to process calls. Maybe it
         wasn't properly started" errors (500). Reported by several persons
         including David Fuchs, Danny Leshem, Yunwu Zhu and Bjorn Roche.
 - Added Readme file
 - Reduce the number of iterators created when internal NIO connector is idle.Reported by Yan Zhou
Merge branch 'master' of git@github.com:restlet/restlet-framework-java
Added HTML escape and unescape features.
Fixed potential security issue that allows Javascript injection inside entities of error reponses. Reported by Danny Leshem.
Merge branch 'master' of git@github.com:restlet/restlet-framework-java
Set ACCEPT header on OAUTH request.

The code that handles the response expects JSON, so we make sure to set
the type to JSON in the ACCEPT header. This makes the code work with
github OAUTH: http://developer.github.com/v3/oauth/.
Merge branch 'master' of github.com:restlet/restlet-framework-java
 - Fixed Javadocs
Updated Windows installer.
Merge branch 'master' of git@github.com:restlet/restlet-framework-java