Parse anonymous sequence containers

[veewee]

Q	A
Type	bug
BC Break	no
Fixed issues

Summary

Fixes

• Support nested sequences inside of choices #23
• Expected "non-empty-string", got "string" phpro/soap-client#506

<xs:complexType>
    <xs:choice minOccurs="0">
        <xs:element name="flag" type="xs:boolean" />
        <xs:sequence>
            <xs:element name="mandatory" type="xs:string" />
            <xs:element minOccurs="0" name="optional" type="xs:boolean" />
        </xs:sequence>
    </xs:choice>
</xs:complexType>

 > http://test-uri/:a {
    ?boolean $flag
    ?string $mandatory
    ?boolean $optional
 }

[lstrojny]

Looks good, thank you!