Use existing upstream Cache-Control if present on upstream response

[J3tto]

Fixes #2272

Change maintains the upstream Cache-Control if present in the upstream response. rather than overiding it with the default.

[slsnextbot]

Handler Size Report

No changes to handler sizes.

Base Handler Sizes (kB) (commit 9facd15)

{
    "Lambda": {
        "Default Lambda": {
            "Standard": 1524,
            "Minified": 668
        },
        "Image Lambda": {
            "Standard": 1488,
            "Minified": 800
        }
    },
    "Lambda@Edge": {
        "Default Lambda": {
            "Standard": 1534,
            "Minified": 673
        },
        "Default Lambda V2": {
            "Standard": 1526,
            "Minified": 670
        },
        "API Lambda": {
            "Standard": 634,
            "Minified": 318
        },
        "Image Lambda": {
            "Standard": 1496,
            "Minified": 805
        },
        "Regeneration Lambda": {
            "Standard": 1187,
            "Minified": 546
        },
        "Regeneration Lambda V2": {
            "Standard": 1253,
            "Minified": 573
        }
    }
}

New Handler Sizes (kB) (commit a519f53)

{
    "Lambda": {
        "Default Lambda": {
            "Standard": 1524,
            "Minified": 668
        },
        "Image Lambda": {
            "Standard": 1488,
            "Minified": 800
        }
    },
    "Lambda@Edge": {
        "Default Lambda": {
            "Standard": 1534,
            "Minified": 673
        },
        "Default Lambda V2": {
            "Standard": 1526,
            "Minified": 670
        },
        "API Lambda": {
            "Standard": 634,
            "Minified": 318
        },
        "Image Lambda": {
            "Standard": 1496,
            "Minified": 805
        },
        "Regeneration Lambda": {
            "Standard": 1187,
            "Minified": 546
        },
        "Regeneration Lambda V2": {
            "Standard": 1253,
            "Minified": 573
        }
    }
}