Merge selected fields in ResolveInfo::getFieldSelection()

[jacor84]

Description

Folding a selection set in ResolveInfo needs to preserve all fields from a given type/level. Currently, there is a possibility to overwrite it with a new set of fields. See an example below, which is a new test case, where the sub-selection from the first level2 would be lost.

    query deepMerge {
      level1 {
        level2 { # used here
          scalar1
        }
        level2 { # and here
          scalar2
        }
        scalar1
        scalar2
      }
    }

The solution is to merge selected fields for FieldNode, which is done in the second commit.

Closes #1364

[spawnia]

Thanks, released with https://github.com/webonyx/graphql-php/releases/tag/v15.2.5.