allow projection of fields for documents within arrays

[hpgrahsl]

while the current field projection mechanisms allow to project fields anywhere within nested documents it's not possible to project on fields found within documents which are themselves contained in arrays.

should work like the following based on this structure:

{ "array": [ {"k":123,"v":"abc"}, {"k":234,"v":"cde"}, {"k":345,"v":"def"} ] }

• blacklisting on "array.k"

{ "array": [ {"v":"abc"}, {"v":"cde"}, {"v":"def"} ] }

• whitelisting on "array.k"

{ "array": [ {"k":123}, {"k":234}, {"k":345} ] }

[hpgrahsl]

addressed with PR #18